There are a number of biological and medical applications that are currently impractical due to limitations in cell and particle analysis technology. Examples of such biological applications include battlefield monitoring of known airborne toxins, as well as the monitoring of cultured cells to detect the presence of both known and unknown toxins. Medical applications include non-invasive prenatal genetic testing and routine cancer screening via the detection and analysis of rare cells (i.e., low rate of occurrence) in peripheral blood. All of these applications require an analysis system with the following principal characteristics:                1. high speed measurement;        2. the ability to process very large or continuous samples;        3. high spectral resolution and bandwidth;        4. good spatial resolution;        5. high sensitivity; and        6. low measurement variation.        
Some advances have been made, particularly with the development of spectral based imaging systems that allow for the analysis of large numbers of particles or cells entrained in a flow of fluid.
In particular, a recently developed imaging flow cytometer technology, termed ImageStream™, makes great strides in achieving each of the above noted principle characteristics. These significant advancements in the art of flow cytometery are described in the following commonly assigned patents: U.S. Pat. No. 6,249,341, issued on Jun. 19, 2001 and entitled “Imaging And Analyzing Parameters of Small Moving Objects Such As Cells;” U.S. Pat. No. 6,211,955 issued on Apr. 3, 2001, also entitled “Imaging And Analyzing Parameters of Small Moving Objects Such As Cells;” U.S. Pat. No. 6,473,176, issued on Oct. 29, 2002, also entitled “Imaging And Analyzing Parameters of Small Moving Objects Such As Cells;” U.S. Pat. No. 6,583,865, issued on Jun. 24, 2003, entitled “Alternative Detector Configuration And Mode of Operation of A Time Delay Integration Particle Analyzer;” U.S. patent application Ser. No. 09/989,031 entitled “Imaging And Analyzing Parameters of Small Moving Objects Such As Cells in Broad Flat Flow.”
While the current analysis rate is sufficient for many applications, there are a number of applications in which a cell detection event is extremely rare. Specifically, events can occur at the rate of 1 in 1 million to 1 in 10 million, or even greater in the case of non-invasive fetal chromosome assessment and early cancer detection.
It would be desirable to provide improvements to the imaging of particles in flow to enable such technology to be used in conjunction with such rare detection events. It would be desirable to provide methods to increase the throughput or analysis rate of such imaging systems.